The invention relates to a method of producing a piezoelectric element, in which electrically conducting electrode layers and piezoelectric synthetic material layers are stacked alternately to form a multilayer, which element is provided, on two facing sides, with planar electrodes in such a manner that the alternate electrode layers are electrically connected to one or to the other planar electrode. The invention also relates to a piezoelectric element produced by means of this method.
Using such a piezoelectric element an electrical potential difference can on the one hand be converted into a mechanical deformation. On the other hand subjecting such as element to a mechanical force produces an electric voltage. Piezoelectric elements are used for example in audio equipment (loudspeakers, headphones, microphones, hydrophones, sonar equipment), in strain detectors and in energy transformers.
A method of producing a multilayer piezoelectric element of the type described in the opening paragraph is inter alia disclosed in the PTC-application filed under the international application number WO 88-04475. Therein the production of a piezoelectric element via a co-extrusion process is described. In this process electrode layers of a synthetic resin material and piezoelectric layers are alternately stacked to form a multilayer. Polymethylacrylate is used as the electrode material for the electrode layers. Polyvenylidene fluoride (PVDF) is used as the piezoelectric material for the piezoelectric layers. Planar electrodes are, subsequently deposited onto facing sides in the form of a silver strip. The electrode layers are stacked in the co-extrusion process in such a manner that the consecutive electrodes extend alternately to the one or to the other above-mentioned side of the multilayer.
The prior art method has a number of disadvantages. For example it has appeared in practice that the electrode layers of the known piezoelectric element are rather thick as a direct result of the extrusion process used. This has the disadvantage that in this prior art piezoelectric element, the quantity of active piezoelectric material at a given volume and at a given piezoelectric synthetic resin material (the "volume efficiency") is relatively small. It is not very readily possible to produce, by means of co-extrusion, optimally functioning multilayer piezoelectric elements, with electrode layer thicknesses less than 25 .mu.m and more particularly less than 10 .mu.m.
Applicants have furthermore found that the piezoelectric elements produced by means of the prior art method do not have optimal properties. In particular, the contact resistance between planar electrodes and the electrode layers is rather high. As a result thereof the prior art piezoelectric elements have an unacceptably high heat loss by dissipation, when these elements are subjected to an a.c. voltage. This heat dissipation appears to increase with time.